This project will develop, implement and evaluate a school-based intervention program which prevents obesity among adolescents aged 11-13 by reducing inactivity, increasing vigorous and moderate physical activity and improving dietary behavior to reduce intake of high fat foods and increase intake of fruits and vegetables. A secondary intervention target is improved knowledge related to inactivity, activity and diet. The focus on reducing inactivity and increasing both moderate and vigorous activity in combination with dietary changes in order to prevent obesity is new, and will address key national objectives for the Year 2000 related to nutrition, activity, and obesity. Schools comprised of children from ethnic groups at risk of obesity will serve as the base for implementing and evaluating interventions among 750 intervention subjects and 750 control subjects over two years, utilizing a quasi- experimental design. Differences in the prevalence of obesity will be assessed through Body Mass Index and triceps skinfold, changes in activity and dietary intake using the Youth Activity and Food Frequency Questionnaires. A process evaluation will document the extent to which environmental changes in school food service and activity resources were implemented. The classroom curriculum consists of 20 lessons of 45 minutes duration which will be given in each of two school years, and represents athematic unit taught in social studies, language arts, math and science classes. The physical education component includes the Physical Best testing materials, topics and activities for the instructor, and a fitness folder which records tests, student activity and inactivity assessments, and goals that students set in consultation with a peer-partner. Classroom activities will be complemented by teacher participation in design and implementation of the intervention, alteration of school food service choices, and participation of school and community activity resources. The programs will be replicable and sustainable, employing low cost curricula in current use in public schools and implementation strategies that tailor intervention materials and activities to existing school personnel. A ten-member Advisory Board for the project includes scientific and professional experts in the epidemiology of obesity, measurement of diet and activity, and the implementation and evaluation of school-based interventions; as well as representatives of the state departments of public health and education that administer and regulate school health and food service activities. The School-Based Obesity Preventive Intervention will result in a program ready for dissemination through public school systems in collaboration with state departments of health and education, and include a package of curricula with documented implementation strategies to tailor the intervention to ethnic composition and health and activity resources of schools and classrooms.